I chose the 150mm for the working distance, yet still retaining a large aperture. Not extending while focusing also sounded like a good idea. I was also keen on a lens with a tripod collar for mounting extra flashes and lights for macro shots.

The lens hasn't disappointed - tack sharp wide open, very fast autofocus (although it can seem slow if you let it search from near to far as it has a long way to go due to the short MFD), takes teleconverters well, is well built and comes with a good quality tripod collar and lens hood.

My only complaint is a general Sigma one, and that's the finish. The finish they use marks easily, and many people report the paint peeling after excessive use on Sigma lenses.

Extremely sharp and great looking results from this, even wide open. Amazingly useful results even with a 2x other than useless AF (even with a non-reporting TC) on a 20D. MF is very smooth and nice to use though.

The AF speed makes this a very enjoyable lens to use for both airshows and birds.

The inbuilt hood is a fantastic design, and gives a lens that is not only very easy to set up and use in a hurry, but also makes it compact enough to fit in a backpack without much trouble.

I'd like the tripod collar foot to be a big larger, for gripping onto it - instead I leave a large QR plate attached, makes hand holding a bit more comfortable.

Stupidly sharp, great colour and contrast, well built, and surprisingly hand-holdable a lot of the time. OOF areas look very nice. Best hood design ever!

Cons:

A longer foot on the tripod collar would have been nice.

The sharp images you get from this wide open is amazing, with the very slightest of improvements when stopped down (mostly in contrast I've found).

Throwing a 2x behind it, despite losing AF usefulness (even with pins taped), made me very surprised indeed. No they're not perfect, but still better than most cheap long lenses without a TC. 1.4x not yet tried.

Have found that the shutter speed needed to stop bird movement motion blur means the lack of IS isn't such an issue as it wouldn't help anyway. Can't really fix the F/5.6 aspect without a whole lot of $$$ and/or weight.

F/4.5-5.6, focusing is a little slow, a little soft at the edges, 77mm filters.

In value-for-money terms I am very happy with this lens. It's nice and light and relatively small (feels like about 1/2 the weight of my Tamron 28-75). Like most reviews I saw it is pin-point sharp in the centre even wide open, I was positively surprised.

On the negative side, it's a somewhat slow lens but you know that when you're buying, and it keeps it small and light. The focusing is a tad slow, but given it's an ultra-wide it doesn't cause an issue (still faster than canon kit lenses). The edges are not razor sharp, but not terrible either. CA towards the edges does show however.

In absolute terms I think it's a "good" lens. At over AU$200 less than any viable alternatives however (i.e. sigma 10-20) I think it's exceptional value.

I got for AU$400 2nd hand, and absolutely love this lens. At F/2.8 I've found it to be very acceptable, only the slightest of softness. From F/3.2 my copy gets very sharp, and stopped down any further it is beyond the resolution of the sensor of my 300D.

The zoom ring being the opposite to canon is a little annoying - I honestly didn't think it'd worry me at all, but it does a bit - but that's not a negative on the lenses part at all. The movement of the zoom ring also isn't silky smooth, it feels a tiny bit tight around 35mm or so, but still quite acceptable.

The Auto/Manual focus switch is fine, the zoom lock is for example better than that on the Canon 70-300 IS USM in my opinion, and the included lens hood fits quite nicely and is nice and strong.